1. Field of the Invention
This invention relates to the rehydration of once-dried fiber for use in a papermaking furnish. Such a rehydration of this types generally, employs temperature, pressure and refining to make the once-dried fiber behave more like a virgin fiber. Thus, higher levels of once-dried fiber can be used without sacrificing paper formation, surface smoothness, and productivity (drainage).
2. Description of the Related Art
It is known that once-dried fiber is stiffer or more rigid than never-dried fiber and can result in hard flocs when used as a significant fraction of the papermaking furnish. These hard flocs deteriorate the formation or uniformity of the resulting paper board and increase the surface roughness of the paperboard.
Once-dried fiber is "hornified" in drying. This hornification closes the lumen pores and reduces the ability of the fiber to rehydrate or pick up water. It is recognized that lumen water increases the flexibility and plasticity of the fiber thereby making it more conformable to adjacent fibers and improves fiber-to-fiber bonding.
High temperature drying of fibers removes the water, both intersticial and lumen, collapses the lumen, and closes lumen pores. Conventional mechanical and chemical fiber treatments have limited success in restoring or rehydrating the lumen and other related fiber properties.
Conventional fiber treatment for once-dried fiber has, typically, been mechanical, such as, deflaking and refining. Deflaking improves once-dried fiber dispersion, but has little impact on rehydration. Refining (conventional and high-consistency) improves rehydration by mechanically working or flexing the fiber and fiber surface. Rehydration by refining is improved, but drainage of the fiber slurry is reduced. Also, strength properties are improved via refining and dispersion is also improved. Increased soak time for once-dried fibers improves ease of dispersion, but has little effect on rehydration.
It is also well known that lumen water content is measured by a technique called "water retention value" or WRV. Typically, this technique subjects a fiber mat to a lengthy (20 minutes), high G-Force centrifuge to remove intersticial water. The lumen water not removed by this technique of centrifuging is an excellent measure of lumen water content and the lumen rehydration of once-dried fiber.
Once-drying reduces WRV by 60-75% of never-dried fiber levels. Unbleached WRV levels are higher than WRV levels of bleached fibers. Also, softwood WRV levels are higher than hardwood WRV levels. This is because the lumen volume of a softwood is larger than the lumen volume of the hardwood.
The deflaking and conventional low consistency (2-5%) refining of once-dried fibers improves or reduces the WRV loss by 10-15% in the same slowness range. Sustained refining (3-6 times conventional virgin fiber slowness) is required to reach virgin fiber WRV levels. Once-dried fiber is also more difficult to refine, thereby, requiring 50-100% of virgin fiber requirements.
Exemplary of such prior art deflaking and refining is U.S. Pat. No. 1,704,533 ('533) to J. A. DeCew, entitled "Hydration by Pump Pressure", and U.S. Pat. No. 1,949,534 ('534) to W. T. Doyle, entitled "Method of and Apparatus for Hydrating Cellulose Pulp". In the '533 patent, there is a discussion of hydration by pump pressure to hydrate fibers by rubbing them against themselves and by mechanical refining. However, the inventor of the present invention has learned that while mechanical refining, particularly gentle fibrillation refining, does result in limited hydration, this is likely the result of rubbing or flexing of the fiber thereby, increasing fiber surface area. Similar limited hydration occurs with refining in virgin (never-dried) fibers. However, once-dried fiber hydration is limited and results in increased drainage resistance during washing/papermaking. Hydration, by even the most gentle refining to virgin fiber WRV levels, results in increased drainage resistance (freeness loss or slowness increase) to such an extent that the resulting fiber may not be desirable because of reduced productivity. Consequently, the hydration improvements of the '533 patent are not from lumen hydration, but the result of increased fibrillation and fine fraction generation.
The '534 patent to Doyle discusses hydration or beating (refining) of fiber in a rotary hammer mill. The process of the Doyle patent does not appear to hydrate the fiber but to "wet" and disperse once-dried fiber sources and fiber flakes/bundles for reuse in papermaking. The dispersion process is comparable to the repulping and deflaking of fiber webs currently employed in the industry in that its intent is to disperse these materials into individual fibers. Deflaking is aided by increased temperature and pH and by chemical addition when the original web contained materials like size or strength additives that inhibit wetting. The processes wet and disperse, not hydrate, fibers. These processes especially do not work on the lumen area of individual fibers. Therefore, a more advantageous rehydration process will be one which would rehydrate the lumen areas of the individual fibers.
It is known that initial ion-stripping and replacement do not improve WRV restoration. Ion stripping and replacement improves energy requirements to restore WRV levels, but drainage loss is still 3-6 fold greater than desired. Also, enzyme hydrolyses of once-dried fiber improves WRV levels approximately 5%. Anti-oxident fiber pretreatment does not affect WRV levels of once-dried fibers.
Finally, it is well known to employ conventional chemical/thermal treatment to improve dispersion, but with little rehydration benefit. Increased pH and elevated temperatures during the mechanical treatment improves dispersion of once-dried fiber significantly, but rehydration is not greatly improved. In fact, digester treatment of once-dried fiber in an alkaline mixture (10-to-1 liquor-to-fiber ratio) at 145.degree. C. produced a yield of 83%.
Exemplary of such prior art is U.S. Pat. No. 2,454,534 ('534) to H. E. Walter, entitled "Process For Defibering Lignocellulose While Subjected to Steam and Alkali-Metal Hydroxide". The process in the Walter patent describes defibering in the presence of a strong alkali. While the patent is described as being effective on lignocellulose, particularly wood chips, it does not mention hydration of once-dried cellulosic fiber, especially once-dried bleached (essentially lignin free) fiber. Also, the Walter patent employs the use of the strong alkali. Therefore, a further advantageous hydration process would be one which would hydrate the lumens of the fiber while avoiding the use of the strong alkali.
It is apparent from the above that there exists a need in the art for a rehydration method which is capable of rehydrating once-dried fiber lumens, but which at the same time avoids the use of conventional cooking liquors.
It is a purpose of this invention to fulfill these and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.